Flashing warning signal



FIG. 1 is a front plan view;

FIG. 2 is a bottom view;

FIG. 3 is a left-side view (the mirror image of the right-side view);

FIG. 4 is a back plan view; and,

FIG. 5 is a top view. 

I claim the ornamental design for a flashing warning signal, as shown and described. 